1. Field of the Invention
The present invention relates to a method for continuous casting of steel using a mold powder applied to a continuous casting mold.
2. Description of the Prior Art
In the continuous casting of steel, a mold powder for the continuous casting mold (referred to "powder" hereinafter) is added adjacent to the steel melt in the mold, and functions to cover the surface of the melt so as to make it retain heat and prevent it from oxidizing. Moreover, the powder is fused by the heat of the hot melt and finds its way into the gap between the inner wall of the mold and the casting where it acts as a lubricant. In addition, the mold is oscillated vertically to prevent sticking between the mold and the casting.
When the properties of the powder material which determine its heat retention, oxidization prevention and lubrication characteristics or other properties of the melting process and the molten-state properties thereof are improper, or the oscillation conditions are improper, various defects, such as surface cracks and slag inclusion occur on the surface of the casting. Consequently, the amount of scarfing required is greatly increased, lowering the production yield, increasing the cost and degrading the quality of the final steel product.
Accordingly, much research has been directed toward optimizing the various properties of the powder and numerous patent applications have been filed on the improvements achieved. In addition, as a matter of fact, many efforts have been made by research engineers at the point of production toward improvement of the vertical oscillation conditions of the mold.
However, the level of the art has still not advanced to the point where a defect-free casting requiring no further processing can be produced. Currently, it is still necessary to subject the whole surface of the casting to auto-scarfing to remove several mm of the skin of the casting, or to partially scarf the surface manually. As the temperature of the casting falls considerably during scarfing, there is a large loss of thermal energy.
More particularly, with increasing casting speed there is an increase both in the occurrence of defects during casting and in the occurrence of trouble such as breakout (caused by molten steel flowing out from the casting) which make it necessary to suspend the continuous casting operation. Because of these problems, the casting speed is restricted.
Quite recently, in order to save energy, increase yield, reduce cost and raise productivity, there has come to be desired technology making it possible to directly charge a hot casting emerging from the continuous casting machine into a heating furnace or to deliver it directly to a rolling machine. For this, it is necessary to be able to produce a steel casting which does not require scarfing or other conditioning treatment.
However, the production of such a casting has up to now been very difficult because the efforts toward improvement have been made separately for the powder and the mold operating conditions.